In the following description the term “turbine” is used to refer to rotary engines having a rotating part and a stator part force coupled by a fluid medium such as water, steam or gas. Of particular interest for the present invention are axial steam turbines comprising radially arranged fixed stator blades or vanes alternating with radially arrangements of moving rotor blades force-coupled by a flow of steam through the turbine.
In large turbines, particularly steam turbines, the starting procedure is considered a very critical operation. A steam turbine can be considered to be large steam turbine for electrical power outputs at full load of 300 MW or more. Typically such turbines include large metal parts such as casings and rotors. As a consequence care has to be taken during the startup of a turbine to not induce for example critical thermal stresses due to the temperature difference between steam fed into the turbine and the large metal parts. Limits on the thermal stresses and other mechanical parameters make the rapid startup of a power plant with a steam turbine section a difficult process.
However there is seen an increasing demand to accelerate the startup time for steam turbines as steam power or combined cycle power plants have to react rapidly to fluctuation in the electrical power supply and demand. Particularly the increase in renewable energy plants such as wind, wave and solar power plants with their fluctuating outputs led to a higher demand for flexibility of the conventional power plants.
It is therefore not surprising that a large body of literature exists contemplating various methods of improving the startup process. Published documents providing a background to the challenges of the startup procedure include for example U.S. Pat. Nos. 3,959,635, 7,506,517, 7,980,053, International Patent Application publication WO 2006/037417, as well as German patent application DE 10116387 (A1), which is owned by the same Assignee as the present application.
In most cases the proposed solution includes a simple or complex feedback control based on measured parameters and a setting of control values based either directly on the measured values or indirectly using models and predictions avoiding the thresholds as defined by the critical stress limits and other parameters. In view of this prior art, it is seen as an object to provide a stable and robust method of controlling the startup of a steam turbine.